1. Field of the Invention
This invention relates to a ceramic insulator coated metal and, more specifically, to a large and thick clad metal member bonded to a relatively thin electrically insulating ceramic material.
2. Brief Description of the Prior Art
Bonding of ceramic materials, such as alumina or the like to metal is generally known in the prior art. Examples of procedures for direct bonding of copper or other metals onto various ceramic materials is set forth, for example, in U.S. Pat. Nos. 3,744,120, 3,766,634, 3,854,892, 3,911,553, 3,993,411, 3,994,430 and 4,129,243, and an article in IEEE Transactions on Components, Hybrids and Manufacturing Technology entitled "Thick Film and Direct Bond Copper Forming Technologies for Aluminum Nitride Substrate" by Nobuo Iwase et al., June, 1985, the subject matter of all of which is incorporated herein by reference.
Where such systems for bonding ceramic and metal materials as described in the above noted patents have been proposed, they have suffered from the limitation that mismatch in the temperature coefficient of expansion (TCE) of ceramic (typically but not limited to alumina, beryllium oxide, aluminum nitride, etc.) and metal (typically copper or high thermal conductivity metals) does not permit relatively large or thick high thermal conductivity metals to be bonded to relatively thin ceramic insulating materials. As an example, the TCE of copper is 16 ppm/oC. whereas the TCE for alumina is 7 ppm/oC., this mismatch resulting in the application of great stresses to the materials with potential cracking of the ceramic material. Accordingly, in accordance with the prior art as exemplified by some of the above noted patents, in order to minimize this problem, the amount of metal proposed for use in conjunction with the ceramic in applications of the type described in the above noted patents is very small. Typically, not more than 0.010 inch copper foil is bonded onto a 0.025 inch thick or thicker alumina member and typically the area or size of the composite has been limited to avoid cracking of the alumina member. In such prior art systems, the core material is generally a relatively thick ceramic material having a metal layer thereon rather than a thick metal core material with the relatively thin layers of ceramic on one or both sides of the metal material.
It is desirable to provide a relatively thick metal (as opposed to the prior art thin metal) core having a relatively thin electrically insulating layer or layers (as opposed to the prior art thick electrically insulating layer) joined thereto in an economical and reliable way whereby the metal core provides the highly beneficial properties of good thermal conductivity by being a good heat sink for components disposed either on the insulating layer or on the metal core itself. This arrangement provides improved rigidity wherein a relatively thick breakable ceramic layer is no longer present, particularly where the metal core has a plurality of separate ceramic sections formed thereon and is capable of providing larger size substrates and wherein the insulating layer will be less subject to cracking due to relatively small thermal mismatch between the metal core and the ceramic layer.